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Area Forecast Discussion

309
FXUS64 KHUN 211042
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
542 AM CDT THU MAY 21 2015

.DISCUSSION...
THE FLOW PATTERN ACROSS NORTH AMERICA THIS MORNING CONTINUES TO
FEATURE A MID-TROPOSPHERIC SUBTROPICAL RIDGE CENTERED ACROSS
MEXICO...WITH LONGWAVE TROUGHS ENCOMPASSING THE PACIFIC COAST AND
EASTERN CANADA. A WEAK 500-MB VORT MAX EMBEDDED WITHIN THE RESULTANT
WEST-NORTHWEST FLOW ACROSS THE TENNESSEE VALLEY WILL CONTINUE TO
SHIFT EASTWARD INTO THE CAROLINAS/GEORGIA THIS MORNING...WITH RELATED
SURFACE COLD FRONT NOW SOUTHEAST OF THE REGION. SHORT TERM MODELS
SUGGEST THAT A TRANQUIL WEATHER PATTERN WILL PERSIST TODAY AND
TONIGHT...AS SUBSIDENT MOTIONS ALOFT RESULT IN GRADUAL MID-LEVEL
HEIGHT RISES. SATELLITE IMAGERY INDICATES A VAST REGION OF LOW
STRATUS CLOUDS UPSTREAM FROM THE LOCAL FORECAST AREA...AND WE ARE
ANTICIPATING CLOUDY SKIES TO PREVAIL THROUGH THE MORNING BEFORE
GRADUAL SCATTERING OCCURS FROM SW-TO-NE THIS AFTERNOON. HIGH
TEMPERATURES WILL BE RATHER COOL AS A RESULT OF THE CLOUD COVER AND A
MODERATE NORTHWEST WIND...WITH VALUES ONLY REACHING THE MID/UPPER
60S. LINGERING CLOUD COVER WILL DISSIPATE BY LATE AFTERNOON/EARLY
EVENING...SETTING THE STAGE FOR STRONG RADIATIONAL COOLING AS WINDS
DIMINISH AND VEER TO THE NORTH-NORTHEAST. LOWS WILL BE UNSEASONABLY
COOL...WITH TEMPERATURES EXPECTED TO FALL INTO THE MID 40S JUST PRIOR
TO SUNRISE. DESPITE LOW DEWPOINTS...THIS REGIME WILL BE FAVORABLE FOR
PATCHY FOG TO DEVELOP IN VALLEYS AND NEAR LARGE BODIES OF WATER AND
THIS HAS BEEN INCLUDED IN THE GRIDS FOR BOTH TONIGHT AND FRIDAY
NIGHT.

DURING THE PERIOD FROM FRIDAY-SATURDAY...A MID-LEVEL VORT IS FORECAST
TO PIVOT NORTHEASTWARD OUT OF THE PACIFIC LONGWAVE TROUGH AND INTO
THE CENTRAL ROCKIES. ALTHOUGH THE STRONGEST FORCING FOR ASCENT AND
RESULTANT PROBABILITIES FOR PRECIPITATION WILL REMAIN CONFINED TO
THE CENTRAL/SOUTHERN GREAT PLAINS...THE EASTERN EXTENT OF THIS
LATITUDINALLY-ORIENTED TROUGH MAY BRING AN INCREASE IN CLOUD COVER TO
THE REGION LATE FRIDAY. A SMALL DISTURBANCE MAY ALSO SPLIT FROM THE
PRIMARY TROUGH AND SHIFT EAST-SOUTHEASTWARD ACROSS THE REGION FRIDAY
NIGHT/SATURDAY MORNING...BRINGING A FURTHER INCREASE IN MAINLY MID
AND HIGH-LEVEL CLOUDS. ALTHOUGH ISENTROPIC PROGS INDICATE A SUBTLE
INCREASE IN ASCENT AROUND THE 300-310K LEVEL AS THE WAVE
APPROACHES...HIGH CPD VALUES AND THE PRESENCE OF A LARGE RESERVOIR OF
DRY AIR BELOW 10-15 KFT WILL SIGNIFICANTLY LIMIT THE THREAT FOR EVEN
LIGHT PRECIPITATION. HOWEVER...A SILENT 10 POP WAS INCLUDED FOR THIS
PERIOD...AS SOME VIRGA OR SPRINKLES MAY OCCUR. MEANWHILE...A STRONGER
VORT MAX IS EXPECTED TO MOVE THROUGH THE BASE OF THE PACIFIC TROUGH
ON TROUGH ON FRIDAY...EJECTING NORTHEASTWARD FROM THE SOUTHWESTERN
DESERTS INTO THE GREAT BASIN BY SATURDAY. THIS WILL AMPLIFY THE
TROUGH AND FORCE THE SUBTROPICAL RIDGE TO GRADUALLY BUILD EASTWARD...
BECOMING CENTERED ACROSS THE FLORIDA PENINSULA BY SATURDAY EVENING.
FURTHER HEIGHT RISES ON SATURDAY WILL TRANSLATE TO WARMER SURFACE
TEMPERATURES WITH HIGHS RISING BACK INTO THE UPPER 70S/LOWER 80S.

GLOBAL MODELS REMAIN IN FAIRLY STRONG AGREEMENT WITH THE OVERALL
PATTERN IN THE EXTENDED PERIOD. GEOPOTENTIAL HEIGHTS ARE FORECAST
TO GRADUALLY RISE WITHIN CENTER OF STRONG SOUTHEASTERN CONUS
ANTICYCLONE WHICH WILL DRIFT NORTHEASTWARD OFF THE COAST OF
GEORGIA/SOUTH CAROLINA EARLY NEXT WEEK. THIS WILL EFFECTIVELY
STRENGTHEN SOUTH- SOUTHWESTERLY GRADIENT FLOW ALOFT OVER THE FORECAST
AREA...WITH THIS TREND ENHANCED FURTHER BY PACIFIC LONGWAVE TROUGH
EJECTING NORTHEASTWARD INTO THE PLAINS. ALTHOUGH THIS WILL LEAD TO A
SUBSTANTIAL INCREASE IN PWAT VALUES...THERE IS LITTLE TO NO EVIDENCE
OF MID-LEVEL FORCING FOR UVM IN OUTPUT FROM THE GFS/ECMWF/GEM.
THUS...WE ARE ONLY EXPECTING A VERY LOW CHANCE OF MAINLY AFTERNOON
AND EARLY EVENING SHOWERS AND STORMS FROM SUNDAY-MONDAY. SUNDAY WILL
LIKELY BE THE WARMEST DAY OF THE PERIOD...WITH SOME PORTIONS OF THE
AREA WEST OF I-65 POSSIBLY APPROACHING 90 DEGREES. GRADUAL MID-
LEVEL HEIGHT FALLS WILL BEGIN ACROSS NW HALF OF THE AREA BY MONDAY
NIGHT AND WILL OCCUR IN CONJUNCTION WITH INCREASE IN PWAT TO 1.5-1.7
INCHES TO BRING A SUBSTANTIALLY HIGHER THREAT FOR NOCTURNAL
CONVECTION...WITH THIS TREND CONTINUING TUESDAY. A FAIRLY WIDESPREAD
NOCTURNAL CONVECTIVE EVENT COULD OCCUR ON TUESDAY NIGHT/WEDNESDAY...
AS ALL GUIDANCE SUGGESTS THAT A WELL-DEFINED WAVE WILL LIFT
NORTHEASTWARD OVER THE REGION -- AND POPS FOR THIS PERIOD MAY NEED TO
BE INCREASED TO AT LEAST THE LIKELY CATEGORY IN FUTURE FORECASTS.
DEEP SHEAR OF 25-30 KNOTS AND CAPE VALUES RANGING FROM 1000-1500 J/KG
ALSO SUGGEST THAT A FEW STRONGER STORMS WILL BE POSSIBLE --
ESPECIALLY TUESDAY/WEDNESDAY.

70/DD

&&

.AVIATION...
/ISSUED 1219 AM CDT THU MAY 21 2015/
FOR 06Z TAFS...MVFR CONDITIONS ARE ANTICIPATED TO PREVAIL UNTIL
21/18Z WITH NW WINDS UP TO 10KTS LIKELY UNTIL 21/15Z. VFR
CONDITIONS SHOULD THEN RETURN BY 21/18Z-21/19Z.

SL.77

&&

.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE    68  48  77  54 /   0   0   0  10
SHOALS        67  48  78  53 /   0   0   0  10
VINEMONT      68  47  76  54 /   0   0   0  10
FAYETTEVILLE  63  44  73  51 /   0   0   0  10
ALBERTVILLE   68  50  75  54 /   0   0   0  10
FORT PAYNE    69  44  76  52 /   0   0   0  10

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.

&&

$$

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.



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